The present invention relates to a method and an apparatus for recording a radio frequency (RF) carrier signal which has been FM modulated with a video signal on a magnetic tape and, more particularly, wherein said invention is suitable for recording an electrical signal, representative of a single frame of a video image, at a time, on a magnetic tape using a dynamic tracking head.
Recordation of RF signals on a magnetic tape is well-known in the art. One application of video tape recording is frame-by-frame recordation of still images to achieve a desired special effect, such as animation.
Heretofore, the method for accomplishing frame-by-frame recording has required the user, after the recordation of one frame, to stop the tape, rewind the tape, stop the tape again, and play the tape in a forward mode until the last recorded frame is reached and then while the tape is still being played, activate the recording head to record the new frame. These repeated steps of stopping, rewinding, and playing forward for each frame cause extreme wear on the tape.
Typically, in a video tape recorder (VTR), RF signals, corresponding to one field, are recorded on one track. The tracks are recorded on the tape as the recording head is rotated, while simultaneously the tape is advanced in a linear direction. The result is that each track recorded on the tape is linear but at an angle to the direction of tape motion, with each track parallel to one another. Two tracks define one frame. All of the foregoing standards are set by the Society of Motion Picture and Television Engineers (SMPTE), and are well-known in the art. These standards apply in the U.S. (NTSC) as well as in Europe.
Because each track is recorded while the tape is moving, if the tape is stopped and a conventional playback head, rotating about an axis by a motor, reads back the previously recorded track, the playback head will not follow the track and the playback signal will diminish in strength towards the end of the field.
A video tape recorder with a dynamic tracking head is well known in the art A dynamic tracking head can be moved by a transducer activated cantilever, operating in response to a tracking control signal, in a direction substantially perpendicular to the direction of tape travel. In addition, the head can be rotated to follow the track of RF signal, as recorded on the magnetic tone.
The function of a dynamic tracking head is to follow the curvature of the track in the playback mode while the tape is stopped or is moving slowly, either in the forward mode or in the reverse mode. This results in faithful reproduction of a still image, while the tape is stopped, and faithful reproduction of a slow motion image, while the tape is moving slowly.
The technique of adjusting the movement of the dynamic tracking head in order for the head to follow the recorded track on the magnetic tape, while the tape is stopped is well-known in the art. One example is to use a feedback signal from the RF section of the VTR to change the tracking control voltage supplied to the cantilever transducer on which the dynamic tracking head is mounted. In the event the RF signal degrades, the tracking control voltage to the cantilever transducer is changed to move the head so that the head will be on track.